Summary: Swaziland and Lesotho have the highest HIV prevalence in the world. They also share another distinct feature: during the last century, they sent a large numbers of migrant workers to South African mines. This paper examines whether participation in mining in a bordering country affects HIV infection rate. A job in the mines means leaving for long periods away from their families and living in an area with an active sex industry. This creates potential incentives for multiple, concurrent partnerships. Using Demographic and Health Surveys, the analysis shows that migrant miners ages 30-44 are 15 percentage points more likely to be HIV positive, and women whose partner is a migrant miner are 8 percentage points more likely to become infected. The study also shows that miners are less likely to abstain or use condoms, and female partners of miners are more likely to engage in extramarital sex. The authors interpret these results as suggesting that miners' migration into South Africa has increased the spread of HIV/AIDS in their countries of origin. Consistent with this interpretation, the association between HIV infection and being a miner or a miner's wife are not statistically significant in Zimbabwe, a country where the mining industry is local and does not involve migrating to South Africa.